Significance and Future Directions
General Conclusions
My research shows that Yellow Warblers wintering in the isthmus of Mexico exhibit sex- but not age-segregation (Chapter 2). As in American Redstarts (Setophaga ruticilla), male Yellow Warblers in the Pacific slope region predominate in limited, wetter habitat (riparian corridors) while females represent a larger proportion of individuals using drier, more abundant, shrub habitat (coastal scrub/seasonal saline marsh). However, sex-segregation appeared to vary by region and was not evident in habitats used by Yellow Warblers on the Gulf of Mexico. Regional differences, if real, may arise because of regional rainfall patterns. Water limitation is a more common within the tropical deciduous forests of the Pacific slope than in the wetter Gulf region. This may produce steeper habitat gradients and possibly greater competition in western Mexico. In both regions wintering Yellow Warblers incorporated local isotopic signatures into their tissues. This was evidenced by significant differences in 13C and 15N between blood samples collected within different habitat types and within different regions. As isotope ratios will be incorporated into feather tissue during pre-alternate moult (which occurs in Yellow Warblers on the wintering grounds (Pyle 1997, pers. observation), winter-grown feathers collected on the breeding grounds can be used as a source of cross-seasonal data.
It is important to note that the inference of winter habitat use from feather isotopes involves several levels of uncertainty. First, in this thesis I assume that isotopic differences in Yellow Warbler tissue reflect winter habitat quality differences. This assumption is based on isotope-habitat relationships reported for other species of
Secondly, “ground-truthed” wintering isotope values in Chapter 3 were obtained from blood samples as most birds captured in Mexico had not completed pre-alternate moult at the time of capture. Blood tends to be 13C and 15N depleted relative to feathers due to tissue-specific differences in isotope discrimination (Caut et al. 2009). For 15N, this means that only relative values can be compared between tissue types. For 13C, blood discrimination values also increase slightly as the diet becomes more 13C depleted (Caut et al. 2009). Feathers do not show this shift in discrimination making comparisons of relative 13C values between tissue types more difficult. Feather signatures should more accurately reflect habitat-13C while 13C differences between habitats presented in Figure 3.1 are likely to be slightly underestimated. Finally, carry-over models assume a linear relationship between wintering isotope signatures and breeding variables. If wintering habitat use is, in fact, a discrete variable (e.g. “high” versus “low quality” habitat) the relationship between habitat, isotope values, and breeding variables may be non-linear. Further work on the wintering grounds is needed to (a) quantify the relationship between 13C, 15N and winter habitat quality for Yellow Warblers and (b) determine species-specific discrimination values for feather and blood samples.
In Chapter 3 I showed evidence for a relationship between winter habitat use (inferred using 13C and 15N in winter-grown feathers), breeding phenology, and subsequent productivity among young (first-breeding season) female Yellow Warblers in Revelstoke, British Columbia. Within this age-sex class, depleted 13C and enriched 15N was associated with earlier clutch initiation dates. This suggested that young females wintering in wetter, and possibly more northern locations were able to breed earlier the following spring. As the timing of clutch initiation was associated with higher productivity, winter habitat use carried-over to impact young female fitness. No similar effects were seen in other age-sex classes.
An equivalent analysis involving Yellow Warblers breeding in Inuvik, Northwest Territories in Chapter 4 yielded less support for winter habitat carry-over effects. Here I found no evidence that 13C or 15N was associated with timing effects in this population. Support existed for a 15N effect on male condition: earlier clutch initiation dates occurred among pairs where males had enriched 15N signatures. However, this effect did not carry-over to influence productivity. Comparisons with the existing
American Redstart data (Norris et al. 2004) and with the Revelstoke data presented in Chapter 3, indicated that Yellow Warblers breeding in Inuvik differ from southern populations in both how they are influenced by winter habitat use and in how the timing of breeding influences their productivity. In total, carry-over effects were weaker in this extreme northern population.
Finally, in Chapter 5 I show strong evidence that wind conditions during the spring migratory period play an important role in the timing of breeding events in Revelstoke and in annual differences in apparent survival. This same analysis provided only weak support for an influence of the wintering and breeding periods on this population. This chapter, and Chapters 3-4 align with recent work (Wilson et al. 2011; LaManna et al. 2012) that suggests a longitudinal difference exists in the contribution of non-breeding conditions to events that occur on the breeding grounds. This is an important avenue for future study. Below, I review the existing research, discuss possible reasons why a longitudinal effect might be expected, and make predictions about how the non-breeding period might impact migrants across North America.
Future directions
Non-breeding effects – are there longitudinal patterns?
The current paradigm for carry-over effects in Neotropical migrants is largely built upon studies of eastern American Redstarts wintering in the Greater Antilles. Here birds in higher quality wintering territories maintain their weight over the wintering period and, in spring, depart earlier than their conspecifics (Marra et al. 1998; Marra and Holmes 2001; Studds and Marra 2005). Similarly, increased food availability in high rainfall years is associated with earlier departure schedules (Studds and Marra 2007; Studds and Marra 2011). Early departing individuals subsequently arrive on the breeding grounds earlier, where they obtain reproductive advantages relative to later- arriving individuals (Norris et al. 2004; Reudink et al. 2009; Tonra et al. 2011; McKellar 2013). However, this pattern does not appear to be universal among passerines breeding in North America (Figure 6.2, Table 6.2). A strong influence of winter
are more strongly supported for species that winter predominantly in the Caribbean (González-Prieto 2012). West of the Rocky Mountains, the minority of studies found support for wintering effects (1:3): two instead found strong evidence of a migratory effect. As briefly discussed in Chapter 5, there are two reasons why this may be the case: (1) eastern migrants likely face different movement costs than western migrants and (2) migrants wintering on islands may face unique constraints relative to mainland individuals.
It is likely that the majority of eastern birds cross ocean during migration, either by island-hopping or by making the uninterrupted 18-40 hr. flight across the Gulf of Mexico (Figure 1 A; Moore and Kerlinger 1987). Flights over water allow no opportunity to rest or refuel: to make the crossing, individuals must be in good physical condition and they must be able to obtain a threshold mass prior to departure (Alerstam 1979; Schaub et al. 2008). Island birds cross open water immediately after departing their winter territories. For these birds, on-territory fattening to a set threshold is likely essential in order to initiate spring movement. Among eastern populations wintering in Central and South America, the ocean barrier is encountered later in migration. For these individuals, on-territory fattening may be less crucial for the initiation of migration, but food availability at pre-barrier stopover sites (such as the Yucatan) may be particularly important. Mainland individuals may also choose to follow an overland (circum-Gulf) route to the breeding grounds (e.g. Stutchbury et al. 2009). This behavioural strategy may reduce dependency on any one fuelling location by allowing birds to make shorter flights. Evidence of lower fat loads carried by migrants at southwestern (fall) stopover sites relative to stopover sites along the Gulf Coast of Mexico would support this (Kelly and Hutto 2005).
Western birds are more likely to use exclusively overland routes (Figure 1A) and, as for mainland eastern populations, on-territory fattening may not be as essential for the initiation of spring movement. While geographical barriers exist in the west in the form of the southwestern deserts, these barriers are not as extreme (birds may rest even if they cannot refuel), they occur later in migration (northern Mexico), and are they interspersed with ‘habitat islands’ such as riparian corridors that are heavily used (Skagen et al. 2005a; Kelly and Hutto 2005). For a subset of western migrants, however, our data indicates that there is a climatological barrier to spring movement.
For individuals moving west of the Sierra Nevada, wind patterns at the altitude of migrant movement oppose north and westward movement (Figure 1B). This would indicate that migration becomes more costly for western birds once they enter the western United States. In contrast, conditions for birds in the central and eastern regions of the US are less hostile with wind vectors facilitating a north-easterly movement (Figure 1B; Appendix A; Gauthreaux 1980).
Taking these geographical factors into consideration, I would make several predictions with respect to longitudinal patterns: (1) for eastern migrant populations wintering in the Caribbean, winter habitat will play a strong role in the timing of movement; (2) for eastern populations that winter in Central and South America, stopover sites before water-crossings will be of greater importance than wintering conditions for timing; (3) for migrants that breed to the west of the continental divide, wind conditions in California will have a stronger influence than wintering conditions on timing. Finally, (4) for both eastern and western populations wintering on the mainland, survival may also be strongly influenced by conditions at stopover sites, which may mask evidence of a wintering-ground effect. No such masking will occur in Caribbean populations because wintering habitat and pre-barrier fuelling sites heavily overlap. By the same token, individual-level carry-over effects in populations that winter on the mainland may stem from access to foraging opportunities at stopover sites, rather than on winter territories.
Further work is needed to test this interpretation. Our data suggests that conditions on migration are correlated with the El Niño Southern Oscillation (ENSO) and are more hostile during La Niña years. Two western studies where timing and productivity shifts could be attributed to wintering conditions might also be interpreted from a migration perspective, as both were correlated with ENSO in the direction we would predict (Nott et al. 2002; Macmynowski et al. 2007). Evidence of a positive relationship between previous-year spring temperatures and American Redstart abundance in the US Northern Rockies Bird Conservation Region (BCR) (Wilson et al. 2011) may also be related to a migration effect. Higher temperatures in this region occur during El Niño years. In this case, a connection with migration conditions is tentative
LaManna et al.’s (2012) work on Swainson’s Thrush (Catharus ustulatus) breeding in the northwestern United States indicated that higher rainfall in the southwestern portion of the spring flyway is linked to higher apparent annual survival. This suggests that wind-speed may not be the only variable influencing populations during migration. I found no support for a rainfall model with Yellow Warblers (Chapter 5). A post-hoc analysis using the regions used by LaManna et al. (2012) also was not competitive with wind models (unpublished data). However, the impact of rain may be species-specific or additive. Higher rainfall is associated with El Niño and therefore reduced wind-speed years; this confound may mask rain effects in the Yellow Warbler system.
An understanding of how non-breeding events alter breeding bird abundance, survival and productivity is needed in order to make effective conservation decisions. Without this knowledge the outcome of management practices on the breeding and wintering grounds will be unpredictable and effort and money may be wasted. If migration is the most costly period for western birds, stopover habitats in the southwestern United States may play a significant role in maintaining western populations. These habitats, limited and threatened by development (Skagen et al. 2005 a,b; van Riper et al. 2008), may need to be prioritized.
References
Alerstam, Thomas. 1979. Wind as Selective Agent in Bird Migration. Ornis Scandinavica 10 (1): 76–93.
Bearhop, Stuart, Geoff M Hilton, Stephen C Votier, and Susan Waldron. 2004. Stable Isotope Ratios Indicate that Body Condition in Migrating Passerines is Influenced by Winter Habitat. Proceedings of the Royal Society B: Biological Sciences 271 (Suppl 4): S215–S218.
Caut, Stéphane, Elena Angulo, and Franck Courchamp. 2009 Variation in discrimination factors (Δ15N and Δ13C): the effect of diet isotopic values and applications for diet reconstruction. Journal of Applied Ecology 46: 443-453.
Gauthreaux, S.A. 1980. The Influence of Global Climatological Factors on the Evolution of Bird Migratory Pathways. Proceedings XVII International Ornithological Congress 17: 517–525.
González-Prieto, Ana María. 2012. Factors Influencing Body Condition and Arrival Phenology of Neotropical Migrants at a Northern Spring Stopover Site. M.Sc., University of Saskatchewan.
Kelly, Jeffrey F., and Richard L. Hutto. 2005. An East-West Comparison of Migration in North American Wood Warblers. The Condor 107 (2): 197–211.
doi:10.2307/4096504.
LaManna, J. A., T. L. George, J. F. Saracco, M. P. Nott, and D. F Desante. 2012. El Nino-Southern Oscillation Influences Annual Survival of a Migratory Songbird at a Regional Scale. The Auk 129 (4): 1–10.
Lincoln, Frederick C., Steven R. Peterson, and John L. Zimmerman. 1998. Migration of Birds. U.S. Department of the Interior, U.S. Fish and Wildlife Service,
Washington, D.C. Circular 16. Jamestown, ND: Northern Prairie Wildlife Research Center Online.
http://www.npwrc.usgs.gov/resource/birds/migratio/index.htm.
Lindsay, Andrea M. 2008. Seasonal Events and Associated Carry-over Effects in a Neotropical Migratory Songbird, the Yellow Warbler (Dendroica petechia). M.Sc., Ohio State University.
Macmynowski, Dena P., Terry L. Root, Grant Ballard, and Geoffrey R. Geupel. 2007. Changes in Spring Arrival of Nearctic-Neotropical Migrants Attributed to Multiscalar Climate. Global Change Biology 13 (11): 2239–2251. doi:10.1111/j.1365-2486.2007.01448.x.
Marra, Peter P. 2012. Studying birds in the context of the annual cycle: Carry-over effects and seasonal interactions. presented at the North American
Ornithological Conference, August, Vancouver, British Columbia.
Marra, Peter P., Keith A. Hobson, and Richard T. Holmes. 1998. Linking Winter and Summer Events in a Migratory Bird by Using Stable-Carbon Isotopes. Science 282 (5395): 1884 –1886. doi:10.1126/science.282.5395.1884.
Marra, Peter P., and Richard T. Holmes. 2001. Consequences of Dominance-Mediated Habitat Segregation in American Redstarts During the Nonbreeding Season. The Auk 118 (1): 92–104.
Mazerolle, Daniel F., Kevin W. Dufour, Keith A. Hobson, and Heidi E. den Haan. 2005. Effects of Large-scale Climatic Fluctuations on Survival and Production of Young in a Neotropical Migrant Songbird, the Yellow Warbler Dendroica petechia. Journal of Avian Biology 36 (2): 155–163. doi:10.1111/j.0908-
8857.2005.03289.x.
Mazerolle, Daniel F., Spencer G. Sealy, and Keith A. Hobson. 2011. Interannual
Flexibility in Breeding Phenology of a Neotropical Migrant Songbird in Response to Weather Conditions at Breeding and Wintering Areas. Ecoscience 18 (1): 18–
McKellar, Ann E. 2013. Winter rainfall predicts phenology in widely separated populations of a migrant songbird. Oecologia, in press. doi: 10.1007/s00442- 012-2520-8
Moore, F., and P. Kerlinger. 1987. Stopover and Fat Deposition by North American Wood-warblers (Parulinae) Following Spring Migration over the Gulf of Mexico. Oecologia 74 (1): 47–54. doi:10.1007/BF00377344.
Norris, D. Ryan, Peter P Marra, T. Kurt Kyser, Thomas W Sherry, and Laurene M Ratcliffe. 2004. Tropical Winter Habitat Limits Reproductive Success on the Temperate Breeding Grounds in a Migratory Bird. Proceedings of the Royal Society of London. Series B: Biological Sciences 271 (1534): 59–64. doi:10.1098/rspb.2003.2569.
Nott, M. Philip, David F Desante, Rodney B Siegel, and Peter Pyle. 2002. Influences of the El Niño/Southern Oscillation and the North Atlantic Oscillation on Avian Productivity in Forests of the Pacific Northwest of North America. Global Ecology and Biogeography 11 (4): 333–342. doi:10.1046/j.1466-822X.2002.00296.x. Pyle, Peter. 1997. Identification Guide to North American Birds, Part I: Columbidae to
Ploceidae. Slate Creek Press.
Reudink, Matthew W, Peter P Marra, T. Kurt Kyser, Peter T Boag, Kathryn M Langin, and Laurene M Ratcliffe. 2009. Non-breeding Season Events Influence Sexual Selection in a Long-distance Migratory Bird. Proceedings of the Royal Society B: Biological Sciences 276 (1662): 1619 –1626. doi:10.1098/rspb.2008.1452. van Riper, Charles, Kristina Paxton, Carena van Riper, Kimberly van Riper, Laura
McGrath, and J. Fontaine. 2008. The Role of Protected Areas as Bird Stop-over Habitat: Ecology and Habitat Utilization by Migrating Land Birds Within Colorado River Riparian Forests of Southwestern North America. Nebraska Cooperative Fish & Wildlife Research Unit -- Staff Publications - Paper 78.
http://digitalcommons.unl.edu/ncfwrustaff/78.
Rockwell, Sarah M., Carol I. Bocetti, and Peter P. Marra. 2012. Carry-Over Effects of Winter Climate on Spring Arrival Date and Reproductive Success in an
Endangered Migratory Bird, Kirtland’s Warbler (Setophaga kirtlandii). The Auk 129 (4): 744–752. doi:10.1525/auk.2012.12003.
Schaub, Michael, Lukas Jenni, and Franz Bairlein. 2008. Fuel Stores, Fuel Accumulation, and the Decision to Depart from a Migration Stopover Site. Behavioral Ecology 19 (3): 657–666. doi:10.1093/beheco/arn023.
Sillett, T. Scott, Richard T. Holmes, and Thomas W. Sherry. 2000. Impacts of a Global Climate Cycle on Population Dynamics of a Migratory Songbird. Science 288 (5473): 2040–2042. doi:10.1126/science.288.5473.2040.
Skagen, Susan K., Jeffrey F. Kelly, Charles van Riper, Richard L. Hutto, Deborah M. Finch, David J. Krueper, and Cynthia P. Melcher. 2005a. Geography of Spring Landbird Migration Through Riparian Habitats in Southwestern North America. The Condor 107 (2): 212–227. Doi:10.1650/7807.
Skagen, Susan K., Rob Hazlewood, and Michael L. Scott. 2005b. The Importance and Future Condition of Western Riparian Ecosystems as Migratory Bird Habitat. USDA Forest Service Gen. Tech. Rep. PSW-GTR-191.
Studds, Colin E, and Peter P Marra. 2007. Linking Fluctuations in Rainfall to
Nonbreeding Season Performance in a Long-distance Migratory Bird, Setophaga ruticilla. Climate Research 35: 115–122. doi:10.3354/cr00718.
Studds, Colin E., and Peter P. Marra. 2005. Nonbreeding Habitat Occupancy and
Population Processes: An Upgrade Experiment with a Migratory Bird. Ecology 86 (9): 2380–2385. doi:10.1890/04-1145.
———. 2011. Rainfall-induced Changes in Food Availability Modify the Spring
Departure Programme of a Migratory Bird. Proceedings of the Royal Society B: Biological Sciences 278 (1723): 3437 –3443. doi:10.1098/rspb.2011.0332. Stutchbury, Bridget J. M., Scott A. Tarof, Tyler Done, Elizabeth Gow, Patrick M. Kramer,
John Tautin, James W. Fox, and Vsevolod Afanasyev. 2009. Tracking Long- Distance Songbird Migration by Using Geolocators. Science 323 (5916): 896– 896. doi:10.1126/science.1166664.
Tonra, Christopher M., Peter P. Marra, and Rebecca L. Holberton. 2011. Migration Phenology and Winter Habitat Quality Are Related to Circulating Androgen in a Long-distance Migratory Bird. Journal of Avian Biology 42 (5): 397–404.
doi:10.1111/j.1600-048X.2011.05333.x.
Wilson, Scott, Shannon L. Ladeau, Anders P. Tøttrup, and Peter P. Marra. 2011. Range-wide effects of breeding- and nonbreeding-season climate on the abundance of a Neotropical migrant songbird. Ecology 92 (9): 1789–1798.
Table 6.1. Overview of cross-seasonal studies involving Neotropical migrant passerines. Study number indicates study location in Figure 6.2.
Figure 6.1 (A) The suggested migratory routes used by Neotropical migrants between temperate and tropical regions (Lincoln 1998) and (B) nighttime wind vectors encountered by migrants during the March- May period (10-year average (2003-2012)). Average vectors were calculated within regions bounded by a longitudinal and latitudinal grid (see Appendix A). Boxed regions indicate the approximate area for which vectors were calculated, but are unadjusted for projection. Arrow length corresponds to wind strength with the exception of (*) where length was halved relative to other vectors in the interest of space.
Figure 6.2 The geographical distribution and main findings of cross-seasonal studies involving Neotropical migrant passerines. Evidence
Appendix A.
Derivation of wind data presented in Figure 6.1
As also described in Chapter 5, wind on migration was derived from modeled climate data extracted from the National Center of Environmental Prediction (NCEP) Reanalysis 1 data archives at the NOAA-CIRES Climate Diagnostics Center at Boulder, Colorado, USA (NOAA NCEP 2012) using the RNCEP program (Kemp et al. 2012). This data has a spatial resolution of 2.5° latitude and longitude and temporal resolution of six hours. Vector data (U- and V-wind speed components (m/s)) were averaged from the 850mb (1500m) and 925mb (700m) level for the March-May period between 2003 and 2012. Noon values were dropped so that the series represented conditions encountered during nighttime migration (between 18:00h and 6:00h). Data was then averaged spatially for regions bounded by the longitudinal and latitudinal limits described below (Table 7.1).
Table 7.1. Region limits and night wind vectors (March-May averages, 2003- 2012) presented in Figure 6.1(b)
Region Latitude °N Longitude °W U-wind (m/s) V-wind (m/s)
South America 5-15 72.5-57.5 -7.2 0.1
Southern Central America 5-15 92.5-75 -3.8 -1.1
Caribbean 17.5-27.5 92.5-72.5 -3.0 1.4 Mexico 17.5-27.5 115-95 0.4 0.6 Southeastern USA 30-40 90-67.5 4.2 1.0 South-central USA 30-40 112.5-93 2.0 2.4 Southwestern USA 30-40 125-115 2.6 -2.8 Northeastern USA 42.5-52.5 90-67.5 2.6 -0.4 North-central USA 42.5-52.5 112.5-93 1.5 0.2 Northwestern USA 42.5-52.5 125-115 2.0 1.1 Northwest Territories 55-65 135-112.5 0.6 1.1 Alaska/Yukon 55-65 160-137.5 -0.9 1.0
References:
Kemp, Michael U., E. Emiel van Loon, Judy Shamoun-Baranes, and Willem Bouten. 2012. RNCEP: Global Weather and Climate Data at Your Fingertips. Methods in Ecology and Evolution 3 (1): 65–70. doi:10.1111/j.2041-210X.2011.00138.x. NOAA NCEP 2012. NCEP/NCAR Reanalysis 1. Earth System Research Laboratory,